A metal electrode coated with lead oxide has been known to be suitable as an electrode for use in electrolysis requiring corrosion resistance or high oxygen overvoltage, for instance, electrolysis for the generation of oxygen, anodic oxidation, electroplating, electrolysis of organic materials, electrolytic treatment of waste water, etc., and various improvements have been made in the electrode. However, since practical problems have still been present, these electrodes have not yet been used generally for industrial applications.
Lead oxide used as the electrode includes two types, that is, rhombic .alpha.-PbO.sub.2 and tetragonal .beta.-PbO.sub.2 of a rutile type structure. While .alpha.-PbO.sub.2 shows poor corrosion reistance when used as an anode for electrolysis as compared with .beta.-PbO.sub.2, .alpha.-PbO.sub.2 with no substantial internal strain can be obtained by electrodepositions when it is electrolytically formed on a metal substrate such as titanium. On the other hand, while .beta.-PbO.sub.2 has good electroconductivity and good corrosion resistance, if .beta.-PbO.sub.2 is electrolytically formed, internal straining due to electrodeposition is generally increased to cause cracking or deteriorate the bondability with the metal substrate.
In addition, these PbO.sub.2 layers are generally poor in mechanical strength, lack processability and passivate the metal substrate, such as titanium, due to the oxidizing effect of PbO.sub.2 thereby making electroconduction difficult.
Among the problems as described above, for improving the bondability between the metal substrate and lead oxide, it has been known to adopt a countermeasure for increasing the surface area of the metal substrate, as described, for example, in Japanese Patent Publication Nos. 31396/83 and 34235/84.
Further, there has also been proposed a method of partially depositing a platinum group metal on a metal substrate by electric discharge as described in Japanese Patent Publication No. 45835/82, and a method of disposing fine noble metal portion areas in a distributed manner on the surface of the substrate as described in Japanese Patent Publication No. 32435/79, for preventing the passivation of the metal substrate. According to these methods, however, a large amount of expensive noble metal is needed, which is not practical and, in addition, they involve complicated procedures.
There have also been many proposals relating to coating a lead oxide layer on a metal substrate by way of various primary layers or intermediate layers. For example, there is a method of previously coating a titanium (IV) compound on the surface of a titanium substrate as described in Japanese Patent Publication No. 45191/78, a method of disposing a thin flash layer of a platinum group metal as described in Japanese Patent Publication No. 9236/81, a method of disposing an intermediate layer made of a platinum group metal or metal oxide as described in Japanese Patent Publication Nos. 30957/83, 31396/83, and 34235/84, a method of disposing an intermediate layer of a carbide and boride of a group IV-VI element and/or silicide of a sub-group of group IV - VI elements and/or silicon carbide as described in Japanese Patent Publication No. 72878/75, and a method of disposing a semiconductor intermediate layer made of a tin compound and an antimony compound as described in Japanese Patent Application (OPI) No. 82680/77 (the term "OPI" as used herein refers to "unexamined published patent application).
Among these methods, the method of disposing the intermediate layer containing the platinum group metal or the oxide thereof is not practical since the intermediate layer itself is extremely expensive. In addition, some of these materials are usually employed as an electrode active substance and, since they show a low oxygen overvoltage as an anode as compared with lead oxide, if electrolytes intrude through pin holes, etc. in the lead oxide coating layer, the intermediate layer acts as an anode to evolve gases due to the electrolytic action at the surface of the intermediate layer to possibly result in peeling and destruction of the lead oxide layer. Further, in the method of disposing an intermediate layer not containing a platinum metal group such as an intermediate layer of a semiconductor material of tin and antimony compounds, although there is less possibility that the intermediate layer will act as an anode, the electroconductivity is insufficient leaving a problem for electric current supply. Further, since the radius of lead ions is 0.78 .ANG. for Pb.sup.4+ (6-coordination), which is greater as compared with 0.69 .ANG. for Sn.sup.4+ or 0.61 .ANG. for Ti.sup.4+, it is difficult to firmly bond the intermediate layer and the lead oxide layer to each other by fusion or by forming a solid-solution. Further, since the .beta.-PbO.sub.2 layer has a great ion radius as described above, considerable stresses occur within it, it being the rutile type oxide, and complete bonding is difficult even to the intermediate layer.
In view of the above, use of .alpha.-PbO.sub.2 with less strain has been proposed and alternate layers of .alpha.-PbO.sub.2 and .beta.-PbO.sub.2 are disclosed in Japanese Patent Publication No. 9472/80. It is also known to apply silver plating to the surface of a metal substrate and dispose .alpha.-PbO.sub.2 further thereover as described in Japanese Patent Publication No. 23947/76. While these methods can provide a lead oxide layer with less strain, there have still been problems such as poor corrosion resistance of .alpha.-PbO.sub.2, solution of silver in an acidic solution, etc., and they can not yet be said to be satisfactory.
As has been described above, known lead oxide-coated electrodes involve various problems in view of their performance and manufacture and no practically excellent electrode had been obtained yet.